1. Field of the Invention
The present invention relates to a photonic bandgap fiber including a hollow core for propagating a light formed along a center axis of a cladding region and air holes periodically arranged around the hollow core.
2. Description of the Related Art
A light in a wavelength range from ultraviolet to visible light (ultraviolet to visible range) is used in a wide range of fields, such as medical care, biotechnology, sensing, measurement, optical recording, and material machining.
From among the above fields, for example, as an application for radiation measurement, an optical waveguide for guiding a light in the ultraviolet range over a relatively long distance is used. More specifically, in facilities such as a nuclear reactor in which radiation is generated, it is a subject of great importance to measure and control intensity of radiation to ensure safety of people and the facilities. Particularly, in the case of performing a regular inspection of the reactor, for example, which necessitates a person to actually go into the reactor to perform an inspection work, it is necessary to measure the intensity of radiation in advance to ensure safety. Therefore, in the case of measuring the radiation in the reactor, a scintillator that converts the radiation into a fluorescence is installed, the fluorescence emitted by the scintillator is guided to a safe location using an optical waveguide, and the amount of a guided fluorescence is measured to obtain the intensity of radiation.
The wavelength of fluorescence emitted by the scintillator is about 0.4 μm to 0.55 μm. As the optical waveguide to guide the light in this wavelength range over a relatively long distance, for example, about 100 m, a plastic optical fiber or a silica-based optical fiber widely used for the field of optical communication is used.
On the other hand, in the field of optical communication using a light in an infrared region, as an optical transmission line, a photonic bandgap fiber (PBGF) is actively researched. The photonic bandgap fiber includes a hollow core for propagating a light formed along the center axis of the fiber and air holes periodically arranged around the hollow core. By forming a Bragg grating with the periodically arranged air holes and creating a bandgap by Bragg reflection from the grating, a light is confined within the hollow core so that the light propagates through the hollow core (see, for example, P. J. Roberts, et al., “Ultimate low loss of hollow-core photonic crystal fibres” Optics Express, v. 13, no. 1, pp. 236-244 (2004) and G. Humbert, et al., “Hollow core photonic crystal fibers for beam delivery” Optics Express, v. 12, no. 8, pp. 1477-1484 (2004)).
As described above, there is a wide range of fields using the light in the ultraviolet to visible range, and thus an optical waveguide capable of guiding the light in this wavelength range with low loss is highly required. However, a current silica-based optical fiber has a high transmission loss of 30 dB/km or more at, for example, a wavelength of 0.4 μm. In addition, the plastic optical fiber has an even higher transmission loss, and therefore there is a problem of limitation in an optical waveguide distance.